Fortunately, Harriers are one of the healthiest of dog breeds, suffering only very rarely from a handful of conditions. One issue that is sometimes seen in the breed is "knuckling over"; this is not really a condition or disease in and of itself, but rather a sign or symptom of some underlying problem. When a dog experiences knuckling over, he begins to drag his hind feet when walking. Toe nails are worn down as are knuckles and the skin on the surface of the hind feet. Sometimes the front legs and feet are affected; the problem first arises, or at least is noticed, in the wrist area. There may be a bending of the foot and/or a bending or bowing of the legs; the front legs often cannot support the weight of the dog. The joints can also start swelling.
Besides Harriers, other breeds have also been known to show signs of knuckling over and doctors are not always immediately sure what causes the problem. They're not completely in the dark, though, and are fairly certain that one of two things can lead to knuckling over in dogs. One is some kind of neurological disease that leads to paralysis. The most likely candidate if your vet suspects a neurological disease is Degenerative Myelopathy; in this disease, the dog's spinal cord slowly dies, causing gradual paralysis. One of the hallmark signs of the disease is the dog dragging his back feet; the rear legs get weaker and weaker and after several weeks they are completely paralyzed. There is unfortunately no treatment for this disease and dogs are often euthanized. A definitive diagnosis of Degenerative Myelopathy can only be made via autopsy, after the dog is dead.
Another, much less serious issue that can lead to knuckling over, and which can be resolved, has to do with nutrition; some experts claim that genetics plays no role, while others say that genetic factors influence the development of knuckling over even in this case. With nutritional causes of knuckling over, the legs of a dog look like they're made of rubber and bend in all sorts of uncomfortable looking positions. Many breeds that grow quickly at the beginning of their lives will develop mild knuckling over as puppies because assimilation of minerals and nutrients may not be able to keep up with the growth rate of the dog. In many breeds, the problem will resolve itself without any outside intervention. In certain cases, though, bones and tissues grow at an uneven rate and the muscles, ligaments and tendons of the legs are not properly formed; this can be caused by imbalances in the dog's diet and missing minerals. Some breeders believe that high protein diets are the culprits, while others believe that the critical element is a highly erratic, unbalanced diet that is not in proportion to the amount of exercise a dog gets. An imbalance in calcium and phosphorous, furthermore, could lead to Rickets in the dog, which is another nutritional cause of knuckling over. You should talk to your vet about the proper diet for your Harrier.